Linux is an attempt at making a free clone of Unix. Initially it aimed to be Unix compatible, though I feel that goal has become less and less important as Linux grew in maturity.
Now all of a sudden we have a complete turn-about as the big Unices want to be like Linux! - Linux is attractive for a variety of reasons, including a fast, well refined kernel, lots of readily available and free applications, good support and, because of these, a growing and loyal following. The utilities available with most Linux distributions are based on the core utilities found in the big Unices and have a large collection of new additions, all working together in a more or less coherent way to build a usable platform.
Nowadays many new Unix administrators have at least some Linux experience and with this background can be easily frustrated when looking for Linux specific utilities (Where is top in Solaris). End users would like to see the applications that used on Linux to run on Unix. And the ability to run Linux on cheap and cheerful PC hardware does not detract from Linux's popularity by any means.
So Sun Microsystems, just like IBM with AIX, finds itself looking at Linux to see what this platform is doing right to make it sucesfull. To me this, more than anything else, is proof that Linux finally grew up.
I expect that a leap-frog game will emmerge between Linux and Unix, particularly Solaris, where the two are competing with innovative features to be the platform of choice for both datacenter and desktop applications.
Congratulations to the Linux community on a job well done.